Mg-containing Zn alloy coatings with highly corrosion-resistant performances demanded mainly in building material applications are extremely hard and brittle. Therefore, coating structures are prone to break during shape processing at metallic coating layers, or interfaces between a metallic coating layer and a steel leading to destructions such as exfoliation, and as the result phenomena called as powdering, in which severe deficiencies of metallic coating layers occur, may be observed. In a case in which powdering occurs, resultant severe decreases in the corrosion resistance itself of coated steels are foreseen.
Improvement of the corrosion resistance of steels by coating the steel surface with a metal such as Zn has been heretofore widely known, and steels coated with Zn, Zn—Al, Zn—Al—Mg, Al—Si, or the like are produced presently on a massive scale. With respect to coated steels, besides corrosion resistance various functions, such as abrasion resistance, are demanded frequently. As a method of coating a steel, a hot dip metal coating method is most widely used.
This is because hot dip metal coating is suitable for mass production, and by performing a processing, such as bending, drawing, and welding, many products may be produced.
The corrosion resistance demanded for a coated steel has been yearly getting higher, and a coating with an Mg content higher than before has been recently proposed as by Patent Literature 1 or Patent Literature 2 below. Such a coating is characterized in that 1) it contains more Mg than a conventional coating, 2) it can exhibit higher corrosion resistance at a planar section than before by regulating the structure, 3) it exhibits corrosion resistance at an end surface, etc., and 4) it has high corrosion resistance even in an atmosphere such as an alkaline atmosphere where a conventional zinc-based or aluminum-based coating has weakness.
In contrast to continuous hot dip metal coating onto steel sheets, techniques of immersion coating (dipping), thermal spraying, vapor deposition, etc. are methods able to coat hard-to-process alloys, because they can be applied to coating of products after processing. Among them, since a thermal spray method is a coating method, by which steels are not dipped in molten metals, has such advantages that there is little thermal influence on steels, or little restriction on the dimension of steels, and the allowable range of the melting point of metals or alloys usable for coating is broad.
A basic method of enhancing the corrosion resistance of coatings is to add Zn to coatings, however in the case of coatings containing solely Zn, the corrosion resistance may be not adequate in many applications. Therefore, an Mg-containing thermal spray coating as described in Patent Literature 3 has been proposed.
The technology is to thermally spray a Zn alloy containing Mg at from 0.3% to 15% to a surface of a steel, and forms a thermally sprayed film superior in corrosion resistance and resistance to scratch. Further, a thermal spray method as a technology for improving corrosion resistance, although limited to a weld, has been proposed as by Patent Literature 4 and Patent Literature 5. The technology according to Patent Literature 4 or Patent Literature 5 is concerning a multilayer-type thermal spray coating containing Zn, Al, Mg, Si, or the like.
Further, in Patent Literature 6 discloses “a thermal spray material superior in explosion proof during thermal spraying and corrosion resistance of a thermally sprayed area, characterized in that in terms of % by mass the Al content is from 13 to 78%, the Ca content is from 1 to 5%, the total content of Al and Ca is 79% or less, and the balance is Mg and unavoidable impurities”.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2008-255464    Patent Literature 2: JP-A No. 2011-190507    Patent Literature 3: Japanese Patent No. 3305573    Patent Literature 4: JP-A No. 2014-208880    Patent Literature 5: JP-A No. 2012-107324    Patent Literature 6: Japanese Patent No. 4757692